Remove idle connections periodically

[isbee]

Related issue: #445

If application executed lot's of concurrent commands due to peak traffic, redis-py creates lot's of connections with upperbound max_connections. The problem is these massive connections are not reaped off by redis-py even though peak traffic is gone.

Currently we can leverage redis node's timeout, so a socket is closed on the server side if it is idle and the client can notice this and close this socket gracefully. But I think this is very low level and tricky because someone can forget or misconfigure timeout on redis, but the impact will be huge if peak traffic occurs.

It'll be better that redis-py support max_idle, min_idle like other redis client(e.g. Lettuce, Jedis, go-redis, ...). So developer can notice this, configure as they needed, reap idle connections automatically by redis-py.